Inter-symbol interference (“ISI”) is an unavoidable consequence of both wired and wireless communications systems. The ones and zero bits of a data stream are converted into an analog signal (thus becoming the “symbols”) for the transmission over a transmission link (also referred to herein as a “communications channel”). Because of the various properties of the medium (of the communications channel) used and the distance of signal travel (through the communications channel), the symbols of the received signal tend to become elongated and smeared into each other. This spreading and smearing of symbols, such that the energy from one symbol affects the next ones in such a way that the received signal has a higher probability of being incorrectly interpreted, is referred to as inter-symbol interference. ISI can be caused by many different reasons, such as filtering effects from hardware or frequency selective fading, from nonlinearities, and from charging effects.
Correspondingly, ISI is a fundamental problem in digital communications in bandwidth-limited links. If the ISI is severe, the received signal quality is poor, and the clock and data recovery circuits tend to fail together.
Take an example of digital data transmitted via a serial communications channel between components within a data processing system. The transmitter on one of the components will serialize the data and send serial differential pair signals by first converting the digital data to an analog signal for sending across the communications channel (the path between the transmitter and receiver, e.g., electrical components, printed circuit board, conductive traces, cables, and so on) to the receiver at the other component within the data processing system. It is the job of the receiver to collect the analog signal, extract a clock from the signal, and de-serialize and digitize the signal. Consider a single pulse (e.g., representing 1 bit) within the transmitted digital data. It starts out at the transmitter as a definite single pulse; however, by traversing the “lossy” channel, it develops “tails” (cursors) on either side of the received pulse. These tails may be caused by losses in the channel resulting from its own inherent resistance of the electrical path, and/or the mismatches between the impedance of the transmitter and receiver ends and the impedance of the channel, which can cause a reflection of a portion of the transmitted signal.
If the communications channel comprises a multimode fiber optic cable, the ISI is the dominant power penalty in the channel power budget and effectively sets the limits for the achievable data rate or transmission distance of the channel, due to the fact that it increases exponentially with the distance and thus, dominates the other penalties in the link power budget. A primary source of ISI in a fiber optic system is signal pulse broadening due to fiber dispersion. There are three types of dispersion in a fiber optic system: modal dispersion, chromatic dispersion, and polarization-mode dispersion. In a multimode fiber, different mode groups have different velocities, which are referred to as modal dispersion. Chromatic dispersion is due to the fact that different wavelengths of light have different velocities.